This application claims the priority of German patent document no. 102008050455.6-55, filed Oct. 8, 2008, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a position indicating process which is suitable for a use in environments that are conducive to multipath propagation of signals.
Satellite navigation systems that are currently available for the purpose of position indicating offer a relatively precise determination of the position of a receiver. However, the receiver requires satellite signals that are as free of interferences as possible. A significant source of such interference, which occurs in certain environments (for example, in cities, inside buildings or in the mountains) is the multipath propagation of satellite signals. In those environments that favor multipath propagation of satellite signals, it is frequently not possible to receive signals without interference, and thus to ensure a precise position indication.
It is therefore an object of the present invention to provide a position indicating process which is particularly suitable for a use in environments that are conducive to multipath propagation of signals.
This and other objects and advantages are achieved by the method and apparatus according to the invention, in which a channel pulse response is determined for the transmission channel of a radio signal (also called a ranging signal) which is provided for position indicating, and which is interfered with by multipath propagation. Based on the channel pulse response, the direct signal path can be estimated, and can then be evaluated for a relatively precise position indication of a receiver of the signal.
In other words, from the plurality of propagation paths of a ranging signal through an environment that fosters multipath propagation, the propagation path which is most likely to correspond to the direct signal path from the transmitter to the receiver can be determined by estimation. As a result, the distance between the transmitter and the receiver can be determined based on signal propagation times. If coordinates of the transmitter are known, with at least four ranging signals from different transmitters (analogous to the satellite navigation), the position of the receiver can be determined very precisely. With a sufficient number of ranging signals, the position computation can make a three-dimensional determination, which permits an indicating of the position, for example, in large multifloor buildings.
According to one embodiment of the invention, a position indicating process includes the following steps:                receiving at least four radio signals emitted by different transmitter stations;        determining a channel pulse response of the transmission channel for each of the received radio signals;        estimating the direct signal path for each of the received radio signals by means of the respectively determined channel pulse response; and        determining the receiving position of the radio signals by evaluating the estimated direct signal paths of the received radio signals.        
The radio signals emitted by the different transmitter stations may be identical except for their carrier frequencies, which may differ by a frequency offset. This simplifies generating the signals, because costly encoding of the radio signals is not required for differentiation by a receiver.
The reception of the radio signals may comprise the following steps:                band-pass filtering of each radio signal;        mixing each band-pass-filtered radio signal down to its corresponding baseband;        low-pass filtering of each mixed-down radio signal;        digitizing each low-pass-filtered radio signal;        transforming each digitized radio signal to the frequency range by means of a fast Fourier transformation (FFT);        extracting individual complex FFT components from the radio signals transformed to the frequency range; and        removing phase shifts from the extracted complex FFT components.        
The channel pulse response may be determined by comparing each received radio signal with an individual undistorted transmitter radio signal copy in the frequency range, and calculating the channel pulse response based on the comparison. For this purpose, replicas of undistorted transmitter radio signals, determined, for example, by measurements, can be filed in a receiver before the start of the operation.
The direct signal path may be estimated by processing the determined channel pulse response using a state space estimation algorithm by means of which the determined channel pulse response is modeled by a finite sum of complex sine functions. Modeling takes place by a parameter estimation in the frequency range.
The direct signal path can be estimated by evaluating the modeled channel pulse response during which the first propagation time component is extracted from the modeled channel pulse response. As a rule, the first propagation time component corresponds relatively precisely to the line-of-sight (LOS) component of the channel pulse response, and therefore to the direct signal path.
The receiving position can be determined by a propagation time difference (Time Difference of Arrival—TDOA) computation using the estimated direct signal paths of each of the received radio signals. In the case of the TDOA, the different arrival times of an impulse emitted isochronously by several transmitters are analyzed for the position indication.
The transmitter stations can emit the radio signals in a synchronized manner. As a result, it can be ensured that TDOA computations lead to meaningful results.
For synchronizing the emission, a pulse-per-second (PPS) signal and/or a 10 MHz signal can be used. A PPS signal is a signal that relatively accurately can signal the start of a second. PPS signals are frequently emitted by precision clocks, such as those of some receivers for satellite navigation signals.
The PPS signal and/or the 10 MHz signal can be continuously readjusted within the phase at a preset repetition rate of (particularly, 10 Hz) in order to ensure a durable synchronization.
According to a further embodiment of the invention, a transmitter station, which is configured to implement the invention and as explained above, and has the following:                a synchronization unit for emitting and receiving synchronization signals to and from respective transmitter stations;        a signal generator for generating a ranging signal; and        a communication module for emitting the ranging signal as a radio signal for the position indication of a receiver.        
Furthermore, an embodiment of the invention provides an arrangement of several transmitter stations according to the invention and as described above, each transmitter station having a line of sight to at least one other transmitter station.
Finally, an embodiment of the invention relates to a position indicating system which is configured to perform the process according to the invention as described above, and has the following:                a bandpass filter for filtering each received radio signal emitted by a transmitter station;        a mixer for mixing down each bandpass-filtered radio signal into its corresponding baseband;        a low-pass filter for filtering of each mixed-down radio signal;        an analog-to-digital converter for digitizing each low-pass-filtered radio signal; and        a processing unit for transforming each digitized radio signal into the frequency range by means of fast Fourier transformation FFT, extracting individual complex FFT components from the radio signals transformed into the frequency range, and removing phase shifts from the extracted complex FFT components.        
The position indicating system can be implemented, for example, in the form of a navigation device. Such a navigation device may also be designed for the reception of navigation signals of a global satellite navigation system. As a result, for example, position indicating in the terrain as well as in buildings, in the inner-city region and in the mountains can be carried out in a reliable and mainly also relatively precise manner.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.